The present invention relates to an image processing circuit and a method for processing an image, and is applicable to, for example, the processing and the recording of the results of imaging with a video camera, an electronic still camera and the like, image displaying on a liquid crystal display and the like, image processing and image synthesis with a personal computer and the like, and image transmission by means of those apparatus.
In conventional various image processing circuits such as an imaging apparatus, various kinds of processing such as recording and reproducing are performed after the dynamic range of an image is compressed.
As for such processing of compressing the dynamic range, there are a method in which the gradation (gray scale) of the whole image is corrected and a method in which the gradation of only the low frequency components of an image is corrected. In the former method, the dynamic range is compressed by a correction of the gradation such as a gamma correction, a knee correction and so-called histogram equalization. On the other hand, in the latter method, the dynamic range is compressed by a correction of the gradation such as the gamma correction and the knee correction.
However, there is a problem that these dynamic range compressing methods are not practically sufficient.
That is, by the method in which the gradation of the whole image is corrected by the gamma correction, the knee correction and the like, the contrast of an object as well as the dynamic range is compressed in a brightness region to be compressed, such as a brightness region including few highlight parts and histograms. Accordingly, the method has a problem that it is difficult to increase the compression rate of the dynamic range and a problem that an image having a low contrast and few variations is generated owing to the compression of the dynamic range.
On the other hand, in the method in which the gradation of only the low frequency components is corrected by the gamma correction, the knee correction and the like, a ratio of the low frequency components to the high frequency components of an image changes owing to the compression of the dynamic range. Owing to this change, the method has a problem that the edges of an image are unnaturally emphasized by the compression of the dynamic range and an image with degraded quality is generated.